osd: fix filestore perf stats update

Update the struct we are about to send, not the (unlocked!) one we will
send the next time around.

Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
osd: move osd_stat into OSDService

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1980 from ceph/wip-osd-stats

osd: move osd_stat tracking into OSDService; fix minor bug

Reviewed-by: Samuel Just <sam.just@inktank.com>